Many equipments for indicating the value of a phenomenon such as pressure or displacement supply an AC or DC drive voltage and include a general purpose output amplifier. It is very simple to use a transducer circuit having a resistive sensing impedance with such equipment. All that is required is a resistive bridge including the resistive sensing impedance of the transducer in one arm thereof. The drive voltage supplied by the equipment, whether AC or DC, is applied across one diagonal of the bridge, and the input of the general purpose amplifier is coupled across the other diagonal. As the value of the resistance of the resistive sensing impedance is varied by pressure or other phenomenon of interest, the bridge is imbalanced so as to cause the amplitude of the voltage across the diagonal connected to the input of the general purpose amplifier to vary in like manner.
For various reasons, however, it is often desirable to use a transducer circuit having a reactive sensing impedance. A reactive sensing impedance requires an AC voltage and, for best operation, it is usually necessary that the frequency of the voltage lie within a narrow band. Transducer circuits have been suggested that have special active means for supplying the AC voltage and for deriving an analog signal proportional to the change in the value of the reactive sensing impedance. Unfortunately, however, the circuits cannot be directly connected to all of the equipments of the type described above because one equipment may supply an AC drive voltage and another may supply a DC drive voltage. Furthermore, the analog signal cannot be directly connected to the general purpose strain gauge amplifier. For these reasons, transducers of the prior art that used reactive sensing impedances are not compatible with equipment designed for use with transducer circuits having a resistive sensing impedance.
In such circuits, a digital signal representing the variations in the value of the reactive sensing impedance may be derived by applying the analog output signal to an A/D converter.
In some situations, it is desirable to provide a digitized signal representing variations in the value of a given phenomenon. Whereas this can be done by deriving an analog signal and applying it to an analog-to-digital converter, the converter adds consderably to the cost.